The present invention relates to a gas re-use system for carbon fiber manufacturing processes based on hydrocarbon thermal decomposition.
The system can relate to or provide for the re-use of gas stemming from the carbon fiber manufacturing process, a process based on the use of an industrial gas as the main raw material.
There can be a feedback pipeline having a force and filtering means to raise the pressure from the reaction furnace output manifold to the input. There are, in turn, return and bleed lines operated independently that assure suitable pressure ranges at the same time both in the reaction furnace feed area and in the extraction area.
This system can have a control means that makes use of mass controllers to adjust the supply of raw materials and the supply of residual gas to keep the gases entering the reaction furnace constant in suitable proportions.
There can be a check made such that the residual gas is practically the same as that of the gas used as raw material.
Carbon nanofibers are filaments of submicron vapour grown carbon fiber (usually known as s-VGCF) of highly graphitic structure which are located between carbon nanotubes and commercial carbon fibers, although the boundary between carbon nanofibers and multilayer nanotubes is not clearly defined.
Carbon nanofibers have a diameter of 30 nm-500 nm and a length of over 1 m.
There is scientific literature available describing and modelizing both the physicochemical characteristics of nanofiber and the generation process at microscopic level from the carbon source used in its production.
These models have been created in most cases on the basis of laboratory experiments making use of controlled atmospheres combined with electron scanning or transmission microscopes
Carbon nanofibers are produced on the basis of catalysis by hydrocarbon decomposition over metal catalytic particles from compounds with metallic atoms, forming nanometric fibrillar structures with a highly graphitic structure.
There are studies, such as those of Oberlin [Oberlin A. et al., Journal of Crystal Growth 32, 335 (1976)], in which the growth of carbon filaments over metallic catalytic particles is analysed by electron transmission microscope.
On the basis of these studies, Oberlin proposed a growth model based on the diffusion of carbon around the surface of the catalytic particles until the surface of the particles is poisoned by an excess of carbon.
He also explained that deposition by carbon thermal decomposition is responsible for the thickening of the filaments and that this process takes place together with the growth process and is therefore very hard to prevent.
For this reason, once the growth period has finished, for instance by poisoning of the catalytic particle, the thickening of the filament is maintained if the pyrolysis conditions continue to exist.
Afterwards, other growth models were put forward that have been considered in the light of experimental data and starting from different simplifying hypotheses that give rise to results to match up to a greater or lesser extent with the observations obtained in the laboratory.
Metal catalytic particles are formed of transition metals with an atomic number between 21 and 30 (Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn), between 39 and 48 (Y, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd), or between 73 and 78 (Ta, W, Re, Os, Ir, Pt). It is also possible to use Al, Sn, Ce and Sb, while those of Fe, Co and Ni are especially suitable.
Different chemical compounds may be used as a source of catalytic metal particles for the continuous production of carbon nanofibers, such as inorganic and organometallic compounds.
There is a significant jump with regard to production method and means from laboratory results to the production of industrial quantities of nanofiber in acceptable conditions from the engineering and economic cost point of view.
On an industrial scale, the ways of preparing metal catalytic particles for feeding into the reaction furnace may be classified in two groups: with substrate and without substrate.
In the former case, when the metal particles are added as substrate, fibers are obtained whose application calls for them to be aligned, as is the case of the use of electron emission sources for microelectronic applications.
In the latter case, also known as floating catalyst, the reaction occurs in a certain volume without the metal particle being in contact with any surface, with the advantage that the nanofibers produced do not have to be separated from the substrate afterwards.
It is very highly improbable that the carbon nanofibers will grow directly from the initial carbon source. It is believed that the filaments appear from side products generated from the thermal decomposition of the initial carbon source.
Some authors state that for light hydrocarbons below C16 any of them may be used without the quality of the nanofiber obtained being dependant on the hydrocarbon selected.
Carbon nanofibers are used for making charged polymers giving rise to materials with enhanced qualities, such as resistance to stress, modulus of elasticity, electrical conductivity and thermal conductivity. Other applications are, for instance, their use in tires in partial replacement of carbon black, or in lithium ion batteries, as carbon nanofibers are readily collated with lithium ions.
When considering the nanofiber growth models, it has been considered that deposition due to carbon thermal decomposition is responsible for the thickening of the filaments produced together with the growth process and that this thickening is maintained if pyrolysis conditions continue to exist. Consequently, in an industrial furnace, thickening continues if the nanofiber is kept in the reactor.
The dwell time of the fibers in the reactor is very important as the longer the dwell time, the larger the diameter of the fibers produced. The dwell time depends on multiple variables connected with the reaction, including the temperature of the furnace, the sizes of the tubes, the pressure gradient, and others. It is advisable to keep the whole system below atmospheric pressure to prevent leaks; however, for their operation the control system and the mass controllers need to work above atmospheric pressure.
The manufacture of nanofibers of this type in industrial processes has been addressed by means of techniques such as that described in the U.S. Pat. No. 5,165,909 incorporated herein by reference, in which use is made of a vertical reactor operating at around 1100° C.
The fiber obtained in this furnace has a diameter between 3.5 and 70 nanometres and a length between 5 and 100 times the diameter.
Regarding the inner structure of the fiber obtained by this procedure, the fiber is made up of concentric layers of ordered atoms and a central area that is either hollow or contains disordered atoms.
The reaction furnace used in this patent is supplied at the top mainly with CO used as the gas with carbon content, a catalyst compound with iron content, and all this in the presence of hydrogen as the diluent gas.
A ceramic filter is situated after the reaction furnace for separating the residual gas and the fiber obtained.
This patent uses a gas residual gas treatment line with a feedback line that comprises a compressor and a small bleed valve, a chemical potassium hydroxide filter to remove the carbon dioxide, and a supply input for enriching the residual gas with carbon monoxide.
The resultant flow divides into two branches: three quarters go to a heat exchanger and from there to the bottom of the furnace to prime the ceramic filter, and the remaining quarter goes to reaction furnace input.
In contrast, the invention can relate to a system for the recirculation of residual gas to the supply, which enables the residual gas from the process to be recirculated and monitors both the feed gases and the pressures required at the reaction furnace input and output.
The special configuration of the system based on the installation of a feedback line leads to a considerable reduction in contamination due to re-use of residual gas.
The result is a lowering of the cost of production through use of less raw material due to the re-use of process output gas.